Set to Django 1.4.2 until askbot is updated to work on 1.5+

If you use Django 1.5.1 you will get this error:

error ImportError: cannot import name str_to_unicode

http://askbot.org/en/question/10016/error-importerror-cannot-import-name-str_to_unicode/

This was blocking me from deploying on Heroku.